This invention relates to method of vapor phase growth for a compound semiconductor.
Recently, vapor phase epitaxial growth methods of III-V group and II-VI group compound semiconductors, especially the metal organic vapor phase epitaxy (MOVPE), hydride vapor phase epitaxy, and chloride vapor phase epitaxy are attracting a wide attention from the aspects of large area epitaxy, mass producibility, controllability of film thickness and composition, and others, and are intensively researched and developed by many manufacturers.
In particular, the production of multi-layer thin film structures of compound semiconductors is drawing a keen interest from the viewpoint of device application. For example, the quantum well structure as shown in FIG. 8A, and the growth method of epitaxial layers having band gap energy which varies continuously in the direction of the film thickness is being energetically researched and developed. In FIGS. 8A and 8B, numeral 201 denotes a GaAs substrate, 202 is a GaAs layer, and 203, 204 are AlGaAs layers.
Conventionally, compound semiconductors having a multi-layer thin film structure represented by the quantum well structure have been grown by molecular beam epitaxy (MBE) methods or by metal organic vapor phase epitaxy (MOVPE) method. Below are explained the AlGaAs/GaAs quantum well structure and AlGaAs layer with a continuously varying Al composition ratio according to the conventional MOVPE method.
FIG. 7 is a gas piping model diagram of a vapor phase growth apparatus used when fabricating a GaAs/AlGaAs multi-layer thin film structure by an MOVPE method, in which numeral 1 is an epitaxial growth reactor, 2 is a substrate, 3 is a carbon-made susceptor, 4 is an rf-coil and 5 is a thermocouple, 61, 62, represent organic metals as source materials for Al, Ga and p-type impurity Zn, that is, trimethyl aluminum [TMA: (CH.sub.3).sub.3 Al], trimethyl gallium [TMG: (CH.sub.3).sub.3 Ga], dimethyl zinc [DMZ: (CH.sub.3).sub.2 ZN); 71, 72 ae hydrogenated gases as source materials for As and N-type impurity Se, that is, arsine (AsH.sub.3), hydrogen selenide (H.sub.2 Se); 81, 82, 83, 84, 85 are mass flow controllers for controlling the flow rate of TMA, TMG, DMZ, AsH.sub.3, H.sub.2 Se; 91, 92, 93, 94, 95 are three-way valves for selectively introducing source gases 61, 62, 63, 71, 72 into either epitaxial growth reactor 1 or exhaust system 120; 100 is a valve; 110 is a mass flow controller; 120 is an exhaust system, and 130 is a scrubber for an exhaust gas.
When growing a quantum well structure shown in FIG. 8A by using this apparatus, first the H.sub.2 flow rate of each line is adjusted by the mass flow controller 110, and while passing the source gases into the exhaust system 120 by the operaton of three-way valves 91, 92, 93, the flow rates of TMA, TMG, AsH.sub.3 are adjusted by mass flow controllers 81, 82, 84 to make stationary, and then by the operation of three-way valves 92, 94, TMG and AsH.sub.3 are introduced into the epitaxial growth reactor 1 to grow a GaAs layer in the first place. Next, by the operation of the three-way valve 91, TMA is further led into the reactor 1 to grow an AlGaAs layer. In succession, by the operation of the three-way valve 91, the supply of TMA is stopped to grow a GaAs layer. In this way, by the operation of the three-way valve 91, one can supply and stop the introduction of TMA into the epitaxial growth reactor 1 repeatedly as many times as required during a specified period, so that a quantum well structure may be produced.
However, in the quantum well structure formed by such method, that is, by changing the source gases, it is difficult to vary the Al composition ratio sharply at the interface of the GaAs layer and AlGaAs layer, and a transient layer with an Al composition varying in the interface is formed with Al being mixed in the GaAs layer. This means a quantum well structure cannot formed with satisfactory reproducibility with the apparaus as designed. This is because TMA is left over in the piping and epitaxial growth reactor when the supply of TMA is stopped by the three-way valve (due to a so-called memory effect), and desired composition changes are not obtained with satisfactory reproducibility.
When growing an AlGaAs layer of which the Al composition ratio is continuously varied in the direction of the thickness of the growth film as shown in FIG. 8B, same as in the case of said growth of a quantum well structure, first the H.sub.2 flow rate of each line is adjusted by the mass flow controller 110, and while passing the source gases to the exhaust system 120 by the operation of three-way valves 91, 92, 94, the flow rates of TMA, TMG, AsH.sub.3 are adjusted by the mass flow controllers 81, 82, 84 to make stationary, and TMG, TMA and AsH.sub.3 are introduced into the epitaxial growth reactor by the operation of the three-way valves 91, 92, 94, thereby starting growth. At this time, the flow of source gas TMA of Al is gradually increased by the mass flow controller 81, and the AlGaAs layer is grown so as to increase the Al composition ratio in the growth film thicknesswise direction.
In such methods, however, since the flow rate of the source gases is gradually varied, the whole flow rate in the epitaxial growth reactor is changed, and since organic metals such as TMA are generally supplied by bubbling to feed source gases, the flow rate change in the mass flow controller is not instantly reflected by the change in the supply quantity of TMA. As a result, it was difficult to form a structure of satisfactory reproducibility by the conventional design of the epitaxial growth reactor.